Alkylation is a reaction in which an alkyl group is added to an organic molecule. Alkylating an isoparaffin with an olefin yields an isoparaffin of higher molecular weight. The alkylation reaction is of particular interest to the petroleum refining industry because C.sub.2 to C.sub.5 olefins can react with isobutane in the presence of an acidic catalyst to produce an isoparaffinic product commonly referred to as alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also because it is free of aromatics and olefins.
Industrial alkylation processes have historically used hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. The sulfuric acid alkylation reaction is particularly sensitive to temperature, with low temperatures being favored to minimize the side reaction of olefin polymerization. Acid strength in these liquid acid catalyzed alkylation processes is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. The hydrofluoric acid process is less temperature sensitive and the acid is easily recovered and purified.
Both sulfuric acid and hydrofluoric acid alkylation share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. Research efforts have been directed to developing alkylation catalysts which are equally as effective as sulfuric or hydrofluoric acids but which avoid many of the problems associated with these two acids. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins" 27 Ind Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
Increasing demands for octane coupled with the increasing environmental concerns have led researchers to seek alkylation processes employing safer, more environmentally acceptable catalyst systems. The goal is to find a commercially viable alternative to the currently used hydrofluoric and sulfuric acid alkylation processes. The following references disclose alkylation catalysts which are alternatives to neat HF.
U.S. Pat. No. 3,862,258 teaches an alkylation process using a catalyst comprising a macroreticular acid cation exchange resin and boron trifluoride. According to the patent, the life of such a catalyst can be extended by the presence in the reaction mixture of closely controlled amounts of water which can be added to the feed as water or as water-forming compound.
U.S. Pat. No. 3,977,621 relates to oligomerization of olefins catalyzed by boron trifluoride which is controlled to yield desired trimer as a dominant lubricant product by adding small amounts of ester together with water or alcohol promoter.
U.S. Pat. No. 4,365,105 also relates to oligomerizing an olefin in the presence of three-component catalyst used in making lubricating oils which comprises a particular silica absorbent with boron trifluoride and water absorbed on the silica.
U.S. Pat. No. 4,394,296 relates to a three-component catalyst used in making lubricating oils which comprises a particular silica absorbent with boron trifluoride and water absorbed on the silica.
U.S. Pat. No. 2,939,890 discloses a process for alkylating an aromatic hydrocarbon with an olefin-acting compound at alkylation conditions in the presence of an alkylation catalyst comprising boron trifluoride-modified alumina. Subsequently, U.S. Pat. No. 3,131,230 discloses the importance of the presence of small amounts of water for maintaining catalyst activity. Both of these patents are limited to aromatic alkylation processes.
U.S. Pat. No. 2,804,491 relates to an isoparaffin-olefin alkylation to make gasoline at temperatures between -20.degree. and 150.degree. F. utilizing a two-component catalyst comprising essentially excess BF.sub.3 with a "silica stabilized gel alumina." No activators are taught.
U.S. Pat. Nos. 3,251,902 and 3,893,942, as well as French Patent 1,593,716 and the article by Kirsh and Potts, DIV. OF PET. CHEM. A.C.S. 15, A109 (1970) address alkylation in the presence of zeolite-based catalyst systems.
U.S. Pat. No. 3,467,728 relates to a process for isomerizing olefinic hydrocarbon, such as 1-butene or 1-pentene by contacting the hydrocarbon with a catalyst comprising a crystalline alumina silicate combined with a substantially anhydrous boron halide.
U.S. Pat. No. 3,800,003 relates to a process for producing an alkylation reaction product from an isoparaffinic reactant and an olefinic reactant containing 1-butene, 2-butene and isobutene which includes passing the olefinic reactant through an isomerization zone. The isomerization catalyst comprises a crystalline aluminosilicate combined with a substantially anhydrous boron halide which can be boron trifluoride. Conventional catalysts are utilized for the alkylation reaction and include sulfuric acid and hydrogen fluoride catalyst which have the disadvantages set forth above.
Catalyst complexes comprising BF.sub.3 as well as BF.sub.3 :H.sub.3 PO.sub.4 adducts have been proposed, and are discussed in greater detail below. While these catalysts effectively overcome many of the safety and environmental drawbacks of sulfuric and hydrofluoric acid alkylation systems, the volume and quality of BF.sub.3 alkylates have not, in the past, proven comparable to that of sulfuric or hydrofluoric acid alkylates. Further, the BF.sub.3 -catalyzed isobutane/butene alkylation processes typically require high isoparaffin/olefin feed ratios of at least about 5:1 to produce an alkylate gasoline product of acceptable quality.
U.K. Patent 545,441, assigned to Standard Oil Development Company, teaches a BF.sub.3 :H.sub.3 PO.sub.4 catalyzed isoparaffin-olefin alkylation process.
U.S. Pat. No. 2,345,095 to Bruner teaches a paraffin-olefin alkylation process catalyzed by a boron trifluoride-water complex, represented by the formula BF.sub.3 :nH.sub.2 O, where n is preferably from 1 to 1.5.
U.S. Pat. Nos. 2,296,370 and 2,296,371 to Slotterbeck disclose a BF.sub.3 :H.sub.2 O:HF catalyst system and an isoparaffin-olefin alkylation process employing the same. The catalyst system is said to avoid yield loss due to oxidation of the resulting alkylate product.
U.K. Patent 550,711 teaches a process for increasing the activity of at least partially spent BF.sub.3 :H.sub.2 O catalyst systems for reuse in an organic condensation reaction. Briefly, the process volatilizes BF.sub.3 from the liquid catalyst mass to the extent required to promote separation of a distinct hydrocarbon phase from the catalyst mass. This hydrocarbon phase is then decanted off and fresh BF.sub.3 is added to restore catalytic activity.
Canadian Patent 424,000 teaches a process for producing gasoline boiling range hydrocarbons from isobutane and a normally gaseous olefin by absorbing the olefin in phosphoric acid of at least 75 weight percent concentration with an amount of isobutane equal to at least three moles of isobutane per mole of alkyl phosphate in the presence of a catalytic mixture of phosphoric acid and boron halide at temperature between 20.degree. C. and 60.degree. C.
U.S. Pat. No. 3,873,634 to Hoffman teaches a method of increasing the rate of ethylene alkylation by isobutane by carrying out the reaction simultaneously with the alkylation of a small amount of a higher weight olefin with isobutane in the presence of a BF.sub.3 :H.sub.3 PO.sub.4 catalyst complex at low temperature and pressure.
U.S. Pat. No. 3,925,500 to Wentzheimer discloses a combined acid alkylation and thermal cracking process employing a BF.sub.3 :H.sub.3 PO.sub.4 acid catalyst in which unconverted propane and ethane from the alkylation process are converted, for example, to propylene and ethylene which are subsequently alkylated with isobutane to evolve a valuable liquid fuel.
U.S. Pat. No. 4,795,728 to Kocal teaches a hydrofluoric acid catalyzed alkylation process for producing motor fuel. The hydrofluoric acid catalyst complex includes from 0.5 to 5 weight percent of a cationic or anionic surfactant component enabling the process to be operated at an olefin:acid volumetric feed ratio of greater than 1.0 while maintaining acceptable alkylate quality.